How do I make multiple things in the background the "ground and have them as "shadow catchers"
You have different options.
Do activate shadow catcher in EACH of the object properties like this in CYCLES:

If you're on EEVEE, setup the shadow catcher MATERIAL to assign to all the objects you want to catch the shadow on, like this:

Alternatively, you can use holdouts to separate what you need but (trimming) the pixels to later compose. Activate the HOLDOUT option in the filter selection, and activate it at COLLECTION level for all your grouped (collection) elements.